If you want to start bicycle commuting the best choice for a bike would be the bike that is in your garage. Although it might not seem perfect it is the easiest and the best way to start. You might need to tune it up and add safety features but it is the most cost effective way to get started. This will also allow you to begin a relationship with one of the good local bike stores in our area. A good bike store is similar to a good friend.
I would ride it for at least six months before considering a new purchase. After that you will have a better idea how to choose your new bike. It is best to love the new bike so take some time riding the old one to find out what you like and want in a bike.